1. Field of the Invention
The present invention relates to a data modulation apparatus, a data modulation method, and a data modulation program, and more particularly, to a data modulation apparatus, a data modulation method, and a data modulation program that can stabilize the recording and reproducing characteristics.
2. Description of the Related Art
When data is transferred through a predetermined transmission line or is recorded on a recording medium such as a magnetic disk, an optical disc, or a magneto optical disc, the data is modulated so as to be appropriate for the transmission line or the recording medium. As one of the modulation methods, block coding is known. In the block coding, a data row is divided into blocks in units (hereinafter, referred to as data words) formed of m×i bits, and the data word is converted into a code word formed of n×i bits in accordance with an appropriate coding rule. Hereinafter, bits of a code word are also referred to as channel bits. When i=1, this code becomes a fixed-length code. On the other hand, when a plurality of “i”s are selectable, in other words, when conversion is performed by selecting a predetermined i from the range of 1 to imax (maximum i), the code becomes a variable-length code. This code that is block-coded is denoted by a variable length code (d, k;m, n;r).
Here, i is referred to as a constraint length, and imax becomes r (maximum constraint length). In addition, d, for example, represents the minimum number of consecutive “0”s interposed between consecutive “1”s, that is, a minimum run of “0”s, and k represents the maximum number of consecutive “0”s interposed between consecutive “1”s, that is, a maximum run of “0”s.
When the code word acquired as described above is recorded on an optical disc, a magneto optical disc, or the like, for example, for a compact disc (CD) or a mini disc (MD) (registered trademark), NRZI (NonReturn to Zero Inverted) modulation is performed in which “1” is for inversion and “0” is for non-inversion based on a variable-length code row, and recording is performed based on an NRZI-modulated variable-length code (hereinafter, referred to as a recording waveform row). This is referred to as mark edge recording. On the other hand, for an ISO-standard magneto optical disc having a size of 3.5 inches and a capacity of 230 MB or the like, the code row for which recording modulation is performed is recorded without being modulated through NRZI modulation. This is referred to as mark position recording. For a recording media having a high recording density that is currently used, the mark edge recording is widely used.
When a minimum inversion interval and a maximum inversion interval of the recording waveform row are Tmin and Tmax, in order to perform high-density recording in the direction of the linear speed, it is preferable that the minimum inversion interval Tmin is increased, that is, the minimum run d is increased. In addition, from the viewpoint of clock recovery, it is preferable that the maximum inversion interval Tmax is decreased, that is, the maximum run k is decreased. In a case where the overwrite characteristics are considered, it is preferable that Tmax/Tmin is decreased. In addition, various modulation methods are proposed and are practically used by considering the conditions of a medium such as significance of an increase in the detection window width Tw=m/n from the viewpoint of Jitter or S/N.
Here, in particular, the modulation methods that are proposed or practically used for an optical disc, a magnetic disk, a magneto optical disc, or the like will be briefly described. An EFM code (represented by (2,10;8,17;1)) that is used for a CD or an MD, an 8-16 code (represented by (2,10;1,2;1)) that is used for a DVD (Digital Versatile Disc), and an RLL (2,7) (represented by (2,7;m, n;r)) that is used for a PD (120 mm and a capacity of 650 MB) are RLL codes having a minimum run d=2. In addition, an RLL (1,7) (represented by (1,7;2, 3;r)) that is used for an MD-DATA2 or an ISO-standard 3.5 inch MO (a capacity of 640 MB) is an RLL code having a minimum run d=1. In addition, an RLL code (Run Length Limited code) having a minimum run d=1 in which the size of the minimum mark or the conversion efficiency is balanced is widely used in a recording and reproducing apparatus of a disc such as an optical disc or a magneto optical disc having a high recording density, which is currently developed and researched.
For example, the modulation table of the variable-length RLL (1,7) code is a table as follows.
TABLE 1RLL(1,7):(d, k;m, n;r) = (1,7;2,3;2)Data PatternCode Patterni = 11100x100100110xi = 20011000 00x0010000 0100001100 00x0000100 010
Here, a symbol x represented in the modulation table is “1” when the next following channel bit is “0”. On the other hand, the symbol x is “0” when the next following channel bit is “1”. Here, the maximum constraint length r is 2.
The parameter of the variable-length RLL (1,7) is (1,7;2, 3,2). Thus, when the bit interval of the recording waveform row is T, the minimum inversion interval Tmin represented by (d+1)·T becomes 2 (=1+1)·T. When the bit interval of the data row is Tdata, the minimum inversion interval Tmin represented by (m/n)×2 becomes 1.33 (=(⅔)×2)·Tdata. In addition, the maximum inversion interval Tmax represented by (k+1)·T is Tmax=8 (=7+1)·T (=(m/n)×8Tdata=(⅔)×8Tdata=5.33 Tdata). Furthermore, the detection window width Tw is represented by (m/n)×Tdata, and the value of Tw=0.67(=⅔) Tdata.
In the channel bit row for which the modulation according to the RLL (1, 7) shown in Table 1 is performed, the occurrence frequency of 2T that is Tmin is the highest, and thereafter the occurrence frequency is higher in order of 3T, 4T, 5T, 6T, . . . . Then, when 2T that is the minimum run (Tmin) is repeated, in other words, when edge information is generated much in a short period, it is frequently advantageous for clock recovery.
For example, when the recording linear density is increased further in the recording and reproducing an optical disc, the minimum run becomes a portion in which error can easily occur. The reason for this is as follows. When a disc is reproduced, the waveform output of the minimum run is smaller than that of other run and can be easily influenced, for example, by defocusing, tangential tilt, or the like. In addition, the reproduction of consecutive minimum mark recording in a high recording linear density can be easily influenced by external disturbances such as noise. Accordingly, error in the data reproduction can easily occur. As an erroneous pattern of data reproduction for such a case, there is a case where a leading edge to a falling edge of the consecutive minimum marks erroneously shifts together. In other words, the occurring bit error length propagates from the start to the end of a section in which the minimum run continues. Accordingly, there is a problem that the propagation of the error is long.
In order to stabilize a case where data is recorded and reproduced with a high linear density, it is effective to limit the continuation of the minimum run.
On the other hand, when data is recorded on a recording medium or data is transferred, coding modulation that is appropriate for the recording medium or the transmission line is performed. However, when a low-frequency band component is included in the modulated code, for example, various error signals such as tracking error in servo control of a disk device may easily change or jitter can easily occur. Accordingly, it is preferable that the low-frequency band component of the modulated code is suppressed possibly as it can be.
As a method of suppressing the low-frequency band component, there is DSV (Digital Sum Value) control. A DSV represents a sum when the channel bit row is allowed to be NRZI (that is, level-coded) so as to be a recording code row and the codes are added as “+1” for “1” of the bit row (symbol of data) and as “−1” for “0” of the bit row. The DSV becomes a reference for a low-frequency band component of the recording code row. By decreasing the absolute value of the positive or negative shake of the DSV, in other words, by performing the DSV control, a DC component of the recording code row is excluded, and accordingly, the low-frequency band component is suppressed.
The DSV control is not performed for the modulated code that is modulated based on the variable-length RLL (1, 7) table shown in Table 1. The DSV control for such a case is implemented by calculating the DSV at a predetermined interval in the coding row after modulation (the channel bit row) and inserting a predetermined DSV bit into the coding row (the channel bit row) (for example, JP-A-11-177431).
The number of DSV bits inserted into the channel bit row is determined on the basis of the minimum run d. When DSV bits are inserted in arbitrary positions within a code word so as to maintain the minimum run for the case of d=1, the necessary number of bits is 2 (=d+1) channel bits. In addition, in order to maintain the maximum run, the necessary number of bits is 4 (=2×(d+1)) channel bits in a case where the DSV bits are inserted in arbitrary positions within a code word. When the DSV control is performed with the number of channel bits less than the above-described number of channel bits, there is a case where it is difficult to perform the DSV control depending on the prior or next pattern.
In the RLL (1, 7) code in which (d, k;m, n)=(1,7;2, 3), when the DSV bits are converted into data together with the conversion ratio, the DSV bits correspond to 4 channel bits×2/3=8/3=2.67 data (2.67 Tdata).
The DSV bits are basically redundant bits. Accordingly, when the efficiency of the code conversion is considered, it is preferable that the number of the DSV bits is decreased possibly as it can be.
In addition, it is preferable that the minimum run d and the maximum run k are not changed by the inserted DSV bits. The reason for this is that changes in (d, k) influence the recording and reproducing characteristics.
However, in the actual RLL code, the minimum run has significant influence on the recording and reproducing characteristics, and accordingly, the minimum run is necessarily maintained. However, the maximum run is not necessarily maintained. Thus, depending on the situations, there is a format in which a pattern that breaks the maximum run is used as a synchronization pattern. The maximum run of the 8-16 code of the DVD (Digital Versatile Disc) is 11T. However, for example, 14T exceeding the maximum run is given in a synchronization pattern portion so as to increase the detection capability of the synchronization pattern.